The present invention relates to the production of fluorinated precursors of superconducting ceramics, and superconducting ceramic powders obtained therefrom. In particular, the invention relates to producing oxide superconductors of the rare earth-alkaline earth metal-cuprate family of superconductors, such as YBa2Cu3Oγ (YBCO) films and coatings.
The discovery of superconducting ceramic powders has fueled a tremendous effort to fabricate these powders into high performance films and coatings. These films and coatings can be used to cover articles, i.e. tapes, wires, wafers, etc., for the fabrication of superconducting end use products.
Current density (JC), also known as critical current, is a key property by which to evaluate the quality of a superconductor. Current density is a vector quantity whose magnitude is the ratio of the magnitude of current flowing in a conductor to the cross-sectional area perpendicular to the current flow. A main challenge in the art is the production of superconducting ceramics which exhibit high current densities.
The precursors of superconducting ceramics are typically grown on supporting materials, termed substrates. Substrates can range from being a single crystal to being a whole article, i.e. tape, wire, etc. The particular substrate upon which a precursor is grown, and the particular conditions such growth takes place, significantly determine the current density that a superconducting ceramic will exhibit.
In general, to achieve a high current density, a precursor is grown on a substrate that allows for oriented crystalline growth, i.e. epitaxial growth, and that primarily produces crystalline superconductors with low angle grain boundaries. (Typically, the precursor and the substrate are chemically dissimilar; thus, the epitaxial growth of the precursor is termed heteroepitaxial.)
Single crystalline structures, and in particular single crystals, allow for better epitaxial growth and thus provide higher current densities vis-à-vis polycrystalline structures. For example, YBCO films, grown on epitaxial single crystals, have exhibited critical current densities of 4×106 amps/cm2 at 77K. In contrast, critical current densities are drastically reduced when YBCO films are grown on polycrystalline structures. In such cases, crystal grain boundaries are, for example, increased thereby producing “weakly linked” crystalline structures.
It has been found that the inclusion of fluorine into the precursors of superconducting ceramics allows for the production of superconducting ceramics with high current densities. It is thought that fluorine enhances the transfer of the crystalline order of a substrate to the growing ceramic precursor, i.e. enhances epitaxial growth.
Prior to the present invention, techniques by which to include fluorine during the growth of ceramic precursors could be broadly divided into two categories. These categories are physical deposition methods and chemical methods.
Physical deposition methods include reactive evaporation, magnetron sputtering, e-beam deposition and laser ablation. Typically, in these methods, fluorine is introduced into a precursor film as barium fluoride. Although these methods form high quality films, these methods require high vacuum environments. Such environments require expensive equipment. Moreover, these methods have very slow formation rates which also adds to the expense and inconvenience of making these precursor films. In addition, physical deposition methods are extremely difficult to scale up to multi-meter lengths which are required for electrical or magnetic applications. Instead, such methods are best suited for thin-film fabrication.
Chemical methods are largely based upon thermally activated chemical reactions of precursor compounds during film formation. A precursor is deposited onto a substrate, and later transformed through chemical reactions. Typically, in these methods, fluorine is introduced into the precursor film as solutions containing trifluoroacetates. Such trifluoroacetates solutions are costly.
An example of a chemical method that is widely used in industry is metalorganic chemical vapor deposition (MOCVD). Ceramic precursor films can be deposited from metalorganic precursors having a high vapor pressure. The precursor film is then heated and converted into the final ceramic in a separate heat treatment.
While MOCVD is a versatile and inexpensive method of film fabrication with potential for high speed production, this method is very sensitive to secondary reactions which may be deleterious to final superconducting properties. For example, in the deposition of materials such as YBa2Cu3Oy, such processes are highly susceptible to the intermediate formation of barium carbonate (BaCO3). High processing temperatures (>900° C.) and extended processing times are required in order to decompose the barium carbonate and obtain the final superconductor. Such extreme reaction conditions result in reactions between the film and the substrate resulting in poor quality films.
Recently, U.S. Pat. No. 5,231,074 issued to Cima et al. described the metal-organic solution deposition (MOD) preparation of Ba2YCu3O7-x, superconductor films having improved electrical transport properties. In this method, the deposition is done using metal trifluoroacetates on a single crystal of SrTiO3 or LaAlO3. Films with thicknesses of about 0.1 μm, and critical current densities of greater than 106 A/cm2 at 77 K were described. However, the superconducting performance of YBa2Cu3O7-x films prepared using this process has been found to depend on film thickness. In particular, electrical performance drops off dramatically as film thickness increases from 0.1 μm to 5.0 μm. Thus, the applications of such processes are limited.
A hybrid process, known as an ex situ process, combines physical deposition and chemical methods. In particular, this process includes the physical deposition of a precursor film which is then processed outside of the physical deposition chamber by conventional chemicothermal processes (Chan et al. Appl. Phys. Lett. 53(15):1443 (October 1988)). This process involves uniform codeposition of CuO, Y2O3, and BaF2 in the correct stoichiometric ratio onto a substrate. The precursor film is then converted under conventional heating conditions into the superconductor by annealing in the presence of water vapor. However, the limitations of physical deposition methods described above remain.
Accordingly, there is a need in the art for a method to produce superconducting ceramic powders, and their precursors, which is more convenient and less costly than current methods. In particular, there is a need for processes that eliminate the high cost of physical deposition. Additionally, there is a need for a fluorination process that uses simple equipment and eliminates the need for expensive trifluoroacetates. Further, there is a need for a method of producing superconducting ceramics in which cracking in the ceramic is reduced. Also, there is a need for a convenient cost effective process by which to produce superconducting ceramics of various thicknesses.